1. Field of the Invention
The present invention relates to a method and an apparatus for carrying out tomographic image processing on image data representing a tomographic image such as a chest CT image and a chest MRI image, and a computer-readable recording medium storing a program to cause a computer to execute the tomographic image processing method.
2. Description of the Related Art
In the field of medicine, various diagnostic image production apparatuses (modalities) using X-rays or the like have been in use, and apparatuses for CR (Computed Radiography), CT (Computed Tomography), MRI (Magnetic Resonance Imaging), and the like have been put into practice. Images generated by these modalities are displayed on CRT displays or output on films by laser printers. In this manner, such images are used for diagnosing areas of diseased tissue and injuries, and understanding degrees thereof in a clinical setting.
CT images and MRI images obtained by CT apparatus and MRI apparatus (hereinafter called tomographic images) have a substantially high density resolution. Therefore, when such an image is reproduced, details thereof can be visualized by setting the contrast high. Meanwhile, a tomographic image of a chest includes lung areas and a mediastinum, and lung areas are in a high density range while a mediastinum is in a low density range.
As has been described above, tomographic images are reproduced by setting the contrast high. However, in a chest tomography image, lung areas and a mediastinum, which are both necessary for diagnosis, are respectively in a high density range and in a low density range, and setting the contrast high narrows a visualization range. As a result, only either the lung areas or the mediastinum have appropriate contrast. This is because the image of either the lung areas or the mediastinum becomes flat if the other has the appropriate density. Therefore, for a chest tomographic image, two images whose tones are respectively converted in order to have lung areas and a mediastinum be in appropriate contrast are reproduced for appropriate diagnosis.
When a windowing condition indicating which range of CT values are displayed in a predetermined display range is considered for a chest CT image, a window level (a center value of the CT values) is set to −600 and a window width (a range of the CT values) is set to around 1500 for lung areas to have the appropriate density. Meanwhile, for a mediastinum, the window level is set to 50 while the window width is set to around 400.
However, regarding a chest tomographic image, it is troublesome for a doctor to carry out diagnosis using two images although the same body portion of the same patient is photographed. Therefore, reproduction of lung areas and a mediastinum in appropriate contrast in one image has been desired. In this case, dynamic range compression processing (as described in U.S. Pat. No. 5,454,044) causing a low density range of an image to have higher density can be carried out as in a conventional CR apparatus. However, in a chest tomographic image, since an edge of a mediastinum in a low density range is extremely sharp, the sharp edge is reproduced to be blurry if the density of the low density range is increased. As a result, a reproduced image becomes substantially unnatural.